[tcp] Remove obsolete constants

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/tcp.h

 /** Smallest port number on which a TCP connection can listen */
 #define TCP_MIN_PORT 1
 
-/* Some IOB constants */
-#define MAX_IOB_LEN	1500
-#define MIN_IOB_LEN	MAX_HDR_LEN + 100 /* To account for padding by LL */
-
 /**
  * Maxmimum advertised TCP window size
  *